1. Field of the Invention
The present invention relates to a mobile communication system and, in particular, to a fast serving cell change method and apparatus in a High Speed Downlink Packet Access (HSDPA) communication system.
2. Description of the Related Art
The Universal Mobile Telecommunications System (UMTS), which is built on the Global System for Mobile Communications (GSM) and the General Packet Radio Services (GPRS) networks with the integration of Wideband Code Division Multiple Access (WCDMA) technology, aims to provide the mobile and computer users with broadband packet-based services including text messaging, voice, and other multimedia services under the universal connectivity concept.
UMTS systems have evolved with the introduction of High Speed Downlink Packet Access (HSDPA) that improves the downlink transfer speed, i.e. the data rate from a base station (Node B) to a User Equipment (UE). In order to secure a stable and fast data transfer speed, HSDPA uses Adaptive Modulation and Coding (AMC) and Hybrid Automatic Repeat reQuest (HARQ). In AMC, the modulation and combining scheme is adaptively selected among Quadrature Phase Shift Keying (QPSK), 16 Quadrature Amplitude Modulation (QAM), and 64 QAM, on a per-user basis. HARQ can be implemented with a short Transmission Time Interval (TTI) and soft combining. That is, when the data is not successfully received and thus the HARQ is triggered between the Node B and the UE, the UE performs soft combining to recover, thereby improving the overall communication efficiency. With respect to the operation of the HARQ, the Node B and UE exchange various control information such as an Orthogonal Variable Spreading Factor (OVSF) code and a block size for the UE, channel quality information for determining the modulation scheme adaptive to the channel quality and modulation scheme information, and channel number and ACKnowledge/Negative AC Knowledge (ACK/NACK) information.
FIG. 1 is a diagram illustrating an asynchronous HSDPA communication system.
In FIG. 1, the asynchronous HSDPA system includes a Core Network (CN) 100, a plurality of Radio Network Subsystems (RNS's) 110 and 120, and a User Equipment (UE) 130.
The RNS 110 includes a Radio Network Controller (RNC) 111 and a plurality of Node B's 113 and 115; and the RNS 112 includes an RNC 112 and a plurality of Node B's 114 and 116. Each Node B is composed of a plurality of cells. Here, the term “Node B” is used interchangeably with the term “base station” for simplification purpose. The RNC is responsible for managing radio resources and can be referred to as a Serving RNC (SRNC), Drift RNC (DRNC), or Controlling RNC (CRNC) according to its role in the network. The SRNC and DRNC are determined according to their roles with respect to the UE, i.e. the SRNC is responsible for managing the information on the UE and communicating data with the CN 100 and the DRNC is responsible for relaying the data between the UE and SRNC. The CRNC is an RNC which controls each Node B. Referring to FIG. 1, when the RNC 111 manages the information on the UE 130, the RNC 111 becomes the SRNC to the UE 130. If the Node B 130 moves such that the data is relayed by the RNC 112, the RNC 112 becomes the DRNC to the UE 130. The RNC 111 controlling the Node B113 becomes the CRNC of the Node B 113.
The RNC and Node B are connected to each other via the lub interface, and the RNCs are connected to each other via the lur interface. The RNC and CN are connected to each other via lu interface. The cell to which the UE is connected is referred to as the serving cell. In FIG. 1, the cell 117 to which the UE 130 is connected is the serving cell of the UE 130. Accordingly, when the UE moves to another cell, the cell to which the user equipment moves and attaches thereto becomes a new serving cell of the UE.
FIG. 2 is a diagram illustrating a serving cell change scenario in the asynchronous HSDPA system of FIG. 1. In the scenario depicted in FIG. 2, a UE 230 connected to the CN 200 via a cell 217 of the Node B 213 controlled by the SRNC 211 moves to the cell 218 of the Node B 214 controlled by the DRNC 212 such that the cell 218 becomes the serving cell of the UE 230. Once the serving cell is changed, the DRNC 212 manages the radio resource of the UE 230, and the SRNC 211 establishes a connection between the UE 230 and the CN 200 via the DRNC 212.
A serving cell change procedure in the conventional HSDPA system is described hereinafter with reference to FIG. 3. FIG. 3 is a message flow diagram illustrating a conventional serving cell change procedure of the HSDPA system.
Referring to FIG. 3, a UE 301 communicates data with the source cell 302 in step S306. The source cell is an old serving cell of the UE before the serving cell change, and the target cell is a new serving cell after the serving cell change. The UE 301 monitors the channel status by measuring the pilot signals broadcasted by the adjacent cells and reports to the RNC 304 the channel status periodically or when the signal strength of either the source cell or target cell is greater than a predetermined threshold value. The measured signal strength is delivered to the RNC 304 via the source cell in the form of a Radio Resource Control (RRC) message in step S308. The RNC 304 determines whether to maintain the source cell as the serving cell of the UE 301 or change the serving cell to the target cell 303 based on the RRC message. Typically, the RNC 304 that determines the serving cell change is the SRNC to the UE 301. When the downlink channel status of the target cell 303 is superior to that of the source cell 302 above the threshold value, the RNC 304 determines a serving cell change from the source cell 302 to the target cell 303. Once it is determined to change the serving cell, the serving cell change procedure is performed, in a sequential order in that the RNC 304 instructs the target cell 303 to prepare the radio link reconfiguration in step S310, the target cell 303 reports the radio link reconfiguration ready to the RNC 304 in step S312, and then the RNC 304 commits the target cell 303 to the radio link reconfiguration in step S314. Next, the RNC 304 sends a radio bearer reconfiguration message to the UE 301 in step S316. At this time, the radio bearer reconfiguration message is delivered to the UE 301 via the source cell 302. With the receipt of the radio bearer reconfiguration message, the UE 301 recognizes that the serving cell change from the source cell 302 to the target cell 303 has been confirmed, thereby completing the serving cell change in step S318 and sending a radio bearer reconfiguration complete message to the RNC 304 in step S320. At this time, the radio bearer reconfiguration complete message is delivered to the RNC 304 via the target cell 303. As a consequence, the UE 301 communicates data with the target cell 303 as a new serving cell in step S324.
In order to secure communication reliability, the serving cell change process should be performed seamlessly with the minimization of transmission latency. Particularly in the case of a real time service such as voice communication, the transmission latency is one of the critical factors to determining the service quality. However, the conventional serving cell change method explained with reference to FIG. 3 has its drawbacks.
One of the drawbacks is caused because the RNC sends the radio bearer reconfiguration message to the UE via the source cell in the format of an RRC message. Despite that the serving cell change from the source cell to the target cell is determined since the channel quality of the target cell is superior to that of the source cell, the RNC sends the radio bearer reconfiguration message via the source cell having relatively bad channel status, whereby the probability of successful receipt of the radio bearer reconfiguration message decreases and, in consequence, causes transmission delay and packet loss.
Also, when the radio bearer reconfiguration message is successfully received, the UE feeds back the radio bearer reconfiguration complete message to the RNC and this feedback process causes further data transmission delay.